(1) Field of the Invention
The present invention generally relates to shifting control apparatus for automatic transmissions of automotive vehicles, and more particularly to a shifting control apparatus for controlling an automatic transmission of an automotive vehicle in which a traction control process is performed so as to prevent the driving wheels of the vehicle from undergoing excessive slippage when the vehicle starts moving or when it is accelerated.
(2) Description of the Related Art
Traction control devices for use in automotive vehicles equipped with automatic transmissions are known. In an automotive vehicle equipped with an automatic transmission, a known traction control process is performed by the traction control device to prevent the driving wheels of the vehicle from undergoing excessive slippage when the vehicle starts moving or when it is accelerated. By performing the traction control process by means of the traction control device, vehicle acceleration and straight line stability are improved when the vehicle is in such operating conditions.
In the above mentioned traction control device, a target rotational speed is predetermined for each of the driving wheels of the vehicle. The slipping amount is defined to be the speed difference between the predetermined target rotational speed and the actual rotational speed of the driving wheels of the vehicle. In the above mentioned traction control device, a feedback control process for the drive torque is performed so as to maintain the slipping amount at a value smaller than a predetermined speed difference value. Hereinafter, the feedback control process for maintaining the slipping amount at a value smaller than a predetermined speed difference value is called the traction control process.
In order to control the drive torque in the feedback control process, the traction control device includes a main throttle valve and a sub-throttle valve provided in the intake passage of the engine. The throttle position of the main throttle valve is adjusted (opened or closed) in accordance with the position of an accelerator pedal set by a vehicle operator. The throttle position of the sub-throttle valve is adjusted by an actuator such as a stepper motor. When slippage occurs in the vehicle, the throttle position of the sub-throttle valve is adjusted by the actuator to reach a smaller throttle position (or, a more closed position) so that the drive torque is reduced. This operation performed by the traction control device is hereinafter called a closing control. Hereinafter, a throttle position of the throttle valve is defined to be the degree to which the throttle valve is open.
Japanese Laid-Open Patent Publication No. 1-218932 discloses a shifting control device for controlling the automatic transmission of the vehicle including the above described traction control device. In the shifting control device disclosed in this publication, a shifting control process for the automatic transmission when the traction control process is not being performed is carried out in accordance with the main throttle position indicated by an output signal of a throttle position sensor provided at the main throttle valve, and in accordance with the vehicle speed indicated by an output signal of a vehicle speed sensor, so that the shifting of the automatic transmission is performed so as to become appropriate for the operating conditions of the vehicle.
In the shifting control device mentioned above, the shifting control process for the automatic transmission when the traction control process is being performed is carried out in accordance with a derived throttle position and the vehicle speed. This throttle position is derived from the estimated air amount Q/N calculated from the intake air amount Q and the engine speed N. The vehicle speed is indicated by an output signal of the vehicle speed sensor. The shifting control process for the automatic transmission is performed substantially in accordance with the throttle position of the sub-throttle valve.
Thus, in the case of the conventional device mentioned above, since the shifting control process is performed in accordance with the throttle position of the sub-throttle valve, it is difficult to suitably perform the shift-down of the automatic transmission if the vehicle operator depresses the accelerator pedal during the traction control process. If the vehicle operator continues to depress the accelerator pedal, the shift-down of the automatic transmission may abruptly take place when the shifting control process in accordance with the throttle position of the sub-throttle valve ends. Accordingly, there is a problem in that the shifting control process performed by the above mentioned conventional device cannot correctly reflect the position of the accelerator pedal set by the vehicle operator when the traction control process is being performed.